Analytical Method – Silicate in Antifreeze Agents and Corrosion Inhibitors

Photometric determination by silicomolybdenum blue analogous to APHA 4500- SiO2 D+E and DIN 38 405-21 and ASTM D859-10

Silicate in antifreeze agents and corrosion inhibitors Corrosion is defined as the transformation of a material resulting from the direct contact with its environment. The corrosion of a material generally leads to an impairment of the proper function of the component made of the material in question. To prevent or at least reduce such damage to a minimum, so-called corrosion inhibitors are used, which are applied to the material to be protected in a suitable way. This can take the form of protective lacquers or additives to the media in direct contact with the material.

Engine coolants for the automobile industry make use inter alia of silicates as corrosion inhibitors. Silicates offer protection in particular to components made of aluminium, which are used e.g. in high-performance engines, by forming a protective layer on the metal1-3. An important factor for the maintenance of this protective layer is to ensure that a sufficient amount of silicate is present in the coolant circuit and accordingly that the silicate concentration is regularly checked at defined maintenance and service intervals. A commonly used method for the detection of silicates is the chemical reaction with molybdate ions. In this reaction, reduction of the silicates at an acidic pH value results in the formation of silicomolybdenum blue, which is then determined by photometry or colorimetry. This principle is used inter alia in official methods for the determination of silicates in drinking water according to the APHA 4500-SiO2 D+E4, DIN 38405-215, and ASTM D859-166 methods.


This Application Note describes the determination of silicate in antifreeze agents and corrosion inhibitors using the Spectroquant® Silicate (Silicic Acid) Test (Cat. No. 114794).

Our Silicate (Silicic Acid) Test enables a simple and swift analysis of the silicate content of coolant liquids to which antifreeze agents and corrosion inhibitors have been added. The method is preprogrammed on the corresponding Spectroquant® photometers. Calibration is not necessary. All reagents required for the measurement are included in the test kit. 300 determinations can be carried out per package.


In sulfuric solution silicate ions react with molybdate ions to form a yellow heteropoly acid. This is reduced to silicomolybdenum blue that is determined photometrically.

The method is analogous to APHA 4500-SiO2 D+E, ASTM D859-10, and DIN 38405-21.

Measuring range

Photometer         Cell             Method No.                Measuring range
      mg/l SiO2       mg/l Si
Prove 100
Prove 300
Prove 600
Prove 60A
50 mm 81 0.011 – 1.600 0.005 – 0.750
20 mm 79 0.11 – 5.35 0.05 – 2.50
10 mm 79 0.21 – 10.70 0.10 – 5.00
Move 100 24 mm 420
0.11 – 8.56 0.05 – 4.00

Sample material

Antifreeze agents and corrosion inhibitors (engine coolants), coolant liquids


Reagents, instruments and materials


Only necessary

if pH adjustment is needed:

For analytical quality assurance:


For the silicate measurement one of the following Spectroquant® photometers is necessary:

Software for data maintenance

The Spectroquant® Prove Connect to LIMS software package provides an easy way to transfer your data into a preexisting LIMS system. This software can be purchased under: Cat. No. Y11086 Prove Connect to LIMS

MQuant® StripScan App

The MQuant® StripScan App is an analytical detection system for the readout of test strips. The app can be downloaded via the IOS app store (the app is currently available for iPhones only). In addition to the app a reference card is necessary which can be purchased under the below-mentioned Cat. No.



For the determination of silicate in engine coolants and antifreeze agents we recommend the use of a 10-mm rectangular cell:

The Move 100 system is supplied with two 24-mm cells. These can be used for the determination of silicate on the Move 100 instrument. Additional cells can be ordered as required:


  • Pipettes for pipetting volumes of 0.50 and 5.0 ml if a 10-mm rectangular cell is used or
  • Pipettes for pipetting volumes of 1.0 and 10.0 ml if a 24-mm cell is used
  • Filters (only necessary for turbid solutions)


Analytical approach

Influence of foreign substances

This was checked individually in solutions containing 8 and 0 mg/l SiO2. The determination is not yet interfered with up to the concentrations of foreign substances given in the table. Cumulative effects were not checked; such effects can, however, not be excluded.

                                              Concentrations of foreign substances in mg/l or %        
1 Mg2+
1000 surfactants* 100
Ca2+ 1000 Mn2+
10 Na-Acetate 10 %
Cd2+ 1000 NH4+ 1000 NaCl 5 %
Cr3+ 100 Ni2+
1000 NaNO3 10 %
Cr2O72- 100 NO2-
1000 Na2SO4 5 %
Cu2+ 10 Pb2+ 10    
10 PO43-
Hg2+ 100 Zn2+

* tested with nonionic, cationic, and anionic surfactants


Sample preparation

  • Analyze immediately after sampling.
  • Before analysis, dilute 1:20 or 1:50 with distilled water1).
  • The pH range must be within 2–10. Check the pH using MQuant® pH indicator strips in combination with the MQuant® StripScan App. Adjust, if necessary, with sodium hydroxide solution 1 mol/l or sulfuric acid 0.5 mol/l.
  • Filter turbid samples.
  • The temperature of the sample should lie between 20–40°C for the analysis. Check the temperature and adjust if necessary.
  • The details regarding the influence of foreign substances on the Spectroquant® Silicate (Silicic Acid) Test must be observed. The stated concentrations of foreign substances should not be exceeded in the diluted sample. If necessary, the sample must be diluted further.

1)The composition of engine coolants is specifically adapted to the type of component and the purpose in question. To better distinguish between engine coolants for different applications, each specific engine coolant contains a dye that gives it its own individual color. To prevent any potential influence of the individual components of the coolant liquid or of the used dye on the silicate determination, before analysis the sample must be diluted 1:20 or, in the case of very high silicate concentrations, 1:50, using distilled water.

When used in cooling systems, the engine coolants are mixed with clean water and poured into the coolant system. The mixing ratio is based on the recommendations of the respective product supplier and the automobile manufacturer. Before an engine coolant is approved, it is tested for its compatibility by the manufacturer using original components of the coolant system.


Preparing the measurement solutions

The following volume details apply for the use of a 10-mm rectangular cell and subsequent measurement on a Spectroquant® Prove 100, Prove 300, Prove 600, or NOVA 60A photometer. When using a 24-mm empty cell and measuring on a Spectroquant® Move 100 colorimeter, the respective volumes must be doubled, since the 24-mm empty cell requires a higher filling volume.

Measurement sample

  • Pipette 5.0 ml pretreated sample into a test tube.
  • Add 3 drops of reagent Si-1 and mix. Hold the bottle vertically while adding the reagent! The pH must be within the range 1.2–1.6. Check pH and adjust, if necessary, with reagent Si-1.
  • Leave to stand for 3 min (reaction time A).
  • Add 3 drops of reagent Si-2 and mix. Hold the bottle vertically while adding the reagent!
  • Add 0.50 ml of reagent Si-3 with pipette and mix.
  • Leave to stand for 10 min (reaction time B), then fill the sample into the cell, and measure in the photometer. The color of the measurement solution remains stable for at least 60 min after the end of the reaction time B stated above.


When using an instrument of the Spectroquant® Prove or Nova series:

  • Insert the barcode cell into the barcode compartment, the method opens automatically. Alternatively, method No. 79 can be selected from the method list.
  • It is recommended to zero the method each new working day. Use the same cell for the zero adjustment as for measuring the sample. For details see the instrument-specific manual.
  • Fill the measurement sample in the 10-mm rectangular cell and insert the cell with the measurement sample into the compartment. The measurement starts automatically.
  • Read off the result for the diluted sample in mg/l from the display. Afterwards multiply the result with the dilution factor to get the results of the original undiluted sample.2,3)

When using a Spectroquant® Move 100 colorimeter:

  • Select method No. 420 from the method list.
  • Fill approx. 10 ml of distilled water into a 24-mm cell (do not add any reagents!), close with the screw cap. (Blank cell)
  • Insert the blank cell into the cell compartment. Align the mark on the cell with that on the photometer. Press <Zero>.
  • Insert the 24-mm cell containing the sample into the cell compartment. Align the mark on the cell with that on the photometer. Press <Test>.
  • Read off the result for the diluted sample in mg/l from the display. Afterwards multiply the result with the dilution factor to get the results of the original undiluted sample.3)

2) An alternative is to enter the dilution factor in the respective method settings. The photometer then automatically integrates the entered dilution into the calculation of the measurement result. For details on entering the dilution factor please refer to the manuals of the individual instruments.

3) In the instrument’s factory settings, the result for this method is calculated and shown in the unit “mg/l” and the citation form “SiO2”. The calculation and display of the result can be switched into the citation form “Si”. For details on how to switch the citation form please refer to the manuals of the individual instruments.


Data transfer Prove spectrophotometers

After measurement transfer the values measured on the Prove spectrophotometer using Prove Connect to LIMS.



Comparison measurements were made between the Spectroquant® Silicate (Silicic Acid) Test and the DIN 38405-21 Method. The samples used for these measurements were a silicate-containing engine coolant and a silicate-free engine coolant. The engine coolants were mixed with tap water of drinking-water quality as per the manufacturer’s instructions, heated to 90°C for 30 minutes in a glass beaker, and subsequently cooled to room temperature. In addition, mixtures of silicate-containing and silicate-free engine coolants as well as samples spiked with silicate were also prepared. The spiked samples were prepared with silicate-free coolant liquid made from distilled water and silicate-free engine coolant and with the addition of defined quantities of silicate. Prior to the analysis by the Spectroquant® Silicate (Silicic Acid) Test, all samples were diluted 1:20 with distilled water. The analysis by the DIN method was performed in undiluted samples.


Silicate Test
Cat. No. 114794
[mg/l SiO2]
DIN 38405-21
[mg/l SiO2]
Tap water 14.1 13.7
Coolant liquid/engine coolant 1
(containing silicate)
90.0 91.8
Coolant liquid/engine coolant 2
(free of silicate)
7.0 6.8
Coolant liquid/ mixture of engine coolants 1+2
74.5 76.1
Coolant liquid/ mixture of engine coolants 1+2
49.5 51.1
Coolant liquid/ mixture of engine coolants 1+2
20.0 18.0
Coolant liquid/engine coolant 2
spiked with 80 mg/l SiO2
77.4 77.0
Coolant liquid/engine coolant 2
spiked with 60 mg/l SiO2
59.2 60.0
Coolant liquid/engine coolant 2
spiked with 25 mg/l SiO2
24.8 27.7
Coolant liquid/engine coolant 2
spiked with 10 mg/l SiO2
10.8 12.0
Silicate standard 80 mg/l SiO2 79.7 80.4
Silicate standard 60 mg/l SiO2 59.2 61.7
Silicate standard 25 mg/l SiO2 24.8 24.8
Silicate standard 10 mg/l SiO2 10.2 10.3


The Spectroquant® Silicate (Silicic Acid) Test delivers ready-to-use, stable reagents for the rapid and economical determination of silicates in engine coolants and antifreeze agents and corrosion inhibitors. The results are comparable with those yielded by the DIN 38450-21 method. The ana­lysis can be performed directly on site without a high investment of instruments.

Moreover, the methods are standard-compliant and preprogrammed on the corresponding Spectroquant® photometers, so there is no need for a time-consuming calibration.

Stringent controls of the raw materials, effective in-process controls and documented final controls are a confirmation for the consistently high quality. Every product batch has an individual certificate, so there is no need to consider checking the quality of the test kit yourself. In addition, the quality certificates confirm consistent batch-to-batch quality.

Analytical Quality Assurance

The objective of analytical quality assurance (AQA) is to secure correct and precise measurement results.

AQA is recommended before each measurement series. To check the measurement system (test reagents, measurement device, and handling) and the mode of working the prediluted silicate standard solutions CRM 1.000 mg/l SiO2 (Cat. No. 132245) can be used. Ready-to-use silicon standard solution Certipur®, concentration 1000 mg/l Si ≙ 2139 mg/l SiO2 (Cat. No. 170236), can also be used after diluting accordingly.

To check for sample-dependent effects the use of addition solutions is highly recommended. For details on how to perform the AQA check see the instrument specific manuals.

A certificate of analysis for each batch as well as a certificate of quality, in which all batches are taken into account, are provided and can be downloaded online on the product page of the test. In the certificates the performance characteristics determined in accordance with ISO 8466-2 and DIN 38402 A51 are given.

Nevertheless, it is recommended to determine the performance characteristic yourself so that all specific factors that may impact the performance (test reagents, measurement device, handling) are considered in the characteristic data.


More information



  1. N.S. Dempster. Corrosion of Aluminum Alloy in Glycol-Water Cooling Systems.
    Corrosion. 1959;15(8):13-16
  2. M. Jayalakshmi, V.S. Muralisharan. Inhibitors for Aluminium Corrosion in Aqueous Media. Corrosion Reviews. 1997;15(3-4):315-340
  3. G. Wildbrett, K. von Grundberr, F. Kiermeier. Zum Verhalten von Natriumsilikat als Korrosionsinhibitor für Aluminium in alkalischen Lösungen. Werkstoffe und Korrosion (Materials and Corrosion). 1967;18(3):217-22
  4. APHA 4500-SiO2 D+E. Standard Methods for the Examination of Water and Wastewater. 2017
  5. DIN 38405-21. Deutsche Einheitsverfahren zur Wasser-, Abwasser- und Schlammuntersuchung; Anionen (Gruppe D); Photometrische Bestimmung von gelöster Kieselsäure (D 21). 1990-10
  6. ASTM D859-16. Standard Test Method for Silica in Water. 2016



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